Add the ability to open a window on a given screen

This is done by supplying the screen in the `WindowOptions` struct.
Note that it's optional, and we will let the operating system choose
which screen to show the window on when `screen` is not provided, as
we did before this change.

crates/gpui/src/platform/mac/platform.rs

@@ -1,10 +1,9 @@
 use super::{
-    event::key_to_native, status_item::StatusItem, BoolExt as _, Dispatcher, FontSystem, Window,
+    event::key_to_native, screen::Screen, status_item::StatusItem, BoolExt as _, Dispatcher,
+    FontSystem, Window,
 };
 use crate::{
-    executor,
-    geometry::vector::{vec2f, Vector2F},
-    keymap,
+    executor, keymap,
     platform::{self, CursorStyle},
     Action, AppVersion, ClipboardItem, Event, Menu, MenuItem,
 };
@@ -14,7 +13,7 @@ use cocoa::{
     appkit::{
         NSApplication, NSApplicationActivationPolicy::NSApplicationActivationPolicyRegular,
         NSEventModifierFlags, NSMenu, NSMenuItem, NSModalResponse, NSOpenPanel, NSPasteboard,
-        NSPasteboardTypeString, NSSavePanel, NSScreen, NSWindow,
+        NSPasteboardTypeString, NSSavePanel, NSWindow,
     },
     base::{id, nil, selector, YES},
     foundation::{
@@ -488,12 +487,11 @@ impl platform::Platform for MacPlatform {
         }
     }
 
-    fn screen_size(&self) -> Vector2F {
-        unsafe {
-            let screen = NSScreen::mainScreen(nil);
-            let frame = NSScreen::frame(screen);
-            vec2f(frame.size.width as f32, frame.size.height as f32)
-        }
+    fn screens(&self) -> Vec<Rc<dyn platform::Screen>> {
+        Screen::all()
+            .into_iter()
+            .map(|screen| Rc::new(screen) as Rc<_>)
+            .collect()
     }
 
     fn open_window(

crates/gpui/src/platform/mac/screen.rs

@@ -0,0 +1,44 @@
+use std::any::Any;
+
+use crate::{
+    geometry::vector::{vec2f, Vector2F},
+    platform,
+};
+use cocoa::{
+    appkit::NSScreen,
+    base::{id, nil},
+    foundation::NSArray,
+};
+
+#[derive(Debug)]
+pub struct Screen {
+    pub(crate) native_screen: id,
+}
+
+impl Screen {
+    pub fn all() -> Vec<Self> {
+        let mut screens = Vec::new();
+        unsafe {
+            let native_screens = NSScreen::screens(nil);
+            for ix in 0..native_screens.count() {
+                screens.push(Screen {
+                    native_screen: native_screens.objectAtIndex(ix),
+                });
+            }
+        }
+        screens
+    }
+}
+
+impl platform::Screen for Screen {
+    fn as_any(&self) -> &dyn Any {
+        self
+    }
+
+    fn size(&self) -> Vector2F {
+        unsafe {
+            let frame = self.native_screen.frame();
+            vec2f(frame.size.width as f32, frame.size.height as f32)
+        }
+    }
+}

crates/gpui/src/platform/mac/window.rs

@@ -8,7 +8,7 @@ use crate::{
     mac::platform::NSViewLayerContentsRedrawDuringViewResize,
     platform::{
         self,
-        mac::{geometry::RectFExt, renderer::Renderer},
+        mac::{geometry::RectFExt, renderer::Renderer, screen::Screen},
         Event, WindowBounds,
     },
     InputHandler, KeyDownEvent, ModifiersChangedEvent, MouseButton, MouseButtonEvent,
@@ -377,11 +377,17 @@ impl Window {
                     msg_send![PANEL_CLASS, alloc]
                 }
             };
-            let native_window = native_window.initWithContentRect_styleMask_backing_defer_(
+            let native_window = native_window.initWithContentRect_styleMask_backing_defer_screen_(
                 RectF::new(Default::default(), vec2f(1024., 768.)).to_ns_rect(),
                 style_mask,
                 NSBackingStoreBuffered,
                 NO,
+                options
+                    .screen
+                    .and_then(|screen| {
+                        Some(screen.as_any().downcast_ref::<Screen>()?.native_screen)
+                    })
+                    .unwrap_or(nil),
             );
             assert!(!native_window.is_null());